The present invention relates generally to security surveillance systems using electromagnetic radiation, and more particularly to the use of microwave radiation to detect contraband hidden inside of a non-metallic container, such as a vehicle tire.
Contraband, such as illegal drugs, explosives, weapons, and cash, is commonly hidden inside of containers to avoid detection when crossing the borders of a country or state. In the United States, a majority of illegal drugs and cash are carried across the border at official ports of entry by automobiles and trucks. Contraband can be hidden inside of false compartments inside of vehicles, such as within gas tanks, behind structural or cosmetic panels, and inside of pneumatic tires. The U.S. Customs Service is tasked with inspecting goods that travel across our borders.
Traditionally, X-rays have been used to provide a two-dimensional snapshot of objects hidden inside of an opaque container. However, X-rays are generally poor at imaging low-density materials, can pose a radiation hazard to the public and operating personnel, and generally require relatively large and expensive equipment. Ultrasonic methods have also been used, but these require the use of a coupling agent (e.g., gel) placed in-between the transducer and the container. Alternatively, thermal neutron analysis has been used, but the equipment is very expensive and large. Handheld detectors (i.e., dosimeters) using gamma rays have been used, but these units have a limited depth of penetration and require using a hazardous radioactive source.
For detecting contraband hidden inside of pneumatic tires, additional techniques have been used, such as: drug-sniffing dogs; striking of the tire sidewalls with hammers or mallets while listening to the acoustic signature; and visual inspection of the tire for unusual signs of wear caused by unbalanced tires packed unevenly with contraband. Other visual clues may be used, such as viewing new or old tires mixed in with the vehicle""s normal set; or dirt (or the lack of it) on selected tires. When these indicators show that there is something unusual about the vehicle or its tires while parked in the primary inspection area, the vehicle is then directed to the secondary stage of inspection, where a second, closer look is taken, tires removed, etc.
Microwave radiation (i.e., 0.5-40 GHz) has been used to detect contraband hidden underneath a person""s clothes and inside of luggage or baggage. See, for example, U.S. Pat. No. 6,057,761 to Yukl; U.S. Pat. No. 5,073,782 to Huguenin, et al.; and U.S. Pat. No. 4,975,968 to Yukl. These systems measure the microwave signal reflected from a sample located inside of an interrogation volume. The unique characteristics of the reflected microwave signal can indicate contraband hidden within. Commonly, a single transceiver unit (i.e., combined transmitter and receiver), is used for operating in the reflection mode. However, a large signal reflected from a thick-walled container, such as the sidewall of a rubber tire, may not carry sufficient information (or have enough sensitivity) to detect a low-density object hidden inside (such as a 5-lb bag of cocaine). Regardless of whether the tire is empty or contains a bag of contraband, the reflected microwave signal is essentially unchanged, which limits its usefulness as a detector.
Microwave radiation has also been used in the transmission mode for characterizing non-metallic objects. By transmission mode, we mean that some of the microwave signal passes completely through the container, which attenuates and slows down (i.e., retards) the microwave. The phase of the microwave can also change when passing through the container. The microwave that passes through the container (i.e., the transmitted microwave) is typically detected by a separate receiver located on the opposite side of the container from the microwave transmitter.
In U.S. Pat. No. 4,607,212 Jakkula describes a method of transmitting pure TEM wave mode microwave radiation through a piece of lumber, and then monitoring the generation of a TM wave mode component in the transmitted wave created by the presence of a knot in the lumber. However, Jakkula does not describe a system for detecting contraband hidden inside of a container.
In U.S. Pat. No. 5,177,444 Cutmore describes a method of measuring the unburnt carbon content of fly ash, comprising passing a microwave through a sample of fly ash and then measuring the attenuation and phase shift of the signal passed through the sample (with respect to the launched signal) to produce a measure of the unburnt carbon content. However, Cutmore does not describe a system for detecting contraband hidden inside of a container.
The measurement of microwave radiation transmitted through an object has also been used to measure the moisture content of wood and paper; to detect the location and depth of steel belts in tire treads; and to monitor the curing rate of rubber used in tire molds, two-part epoxy mixtures, and other polymeric reactions.
The need remains, therefore, for a safe, low power, low-cost, non-destructive contraband-detection method and apparatus that is capable of continuous operation at an official port of entry, year-round, with minimal attention by inspection personnel.
Against this background, the present invention was developed.